Last night, I was re-(re-re-re-re-)reading SCP-3333, and I finally realized why the D-class is named D-4f68a; it's hexidecimal, and is meant to imply that the Foundation has gone through so many D-Class designations that they ran out of numerical ones. Either that, or it's a stylistic choice by the author or is meant to imply an alternate Foundation whose rules are slightly different.

Regardless, it prompted an interesting shower thought: does the Foundation keep records of previous D-class? We know a bit about D-5674, for example, thanks to 860's exploration logs, but doesn't that imply the existence of a D-5673 and -5675? Are there records of them somewhere?

If you ask me, I would say of course they do; the Foundation archives anything and everything that they do and/or are involved with. Except, D-class personnel obviously die during testing (occasionally,) so I don't exactly know why specific authors don't just reuse designations.

For example, let's say D-0304 died in an experiment with SCP-372. After a couple of months, I'm sure that they would recycle the designation D-0304 to another D-class personnel.

If you mean if there is a post on the forums of all the previous iterations of D-class personnel, then the answer is no (to my knowledge). That would be a lot of work.

Why would they recycle a designation? Records keeping things nice and neat and organized would get muddied real fast. It would be easy enough to just keep the numbers going and keep records of D-Class, what testing they do, notes on their abilities/attributes, when/where/how they die, if they are transferred, etc.

If you mean if there is a post on the forums of all the previous iterations of D-class personnel, then the answer is no (to my knowledge). That would be a lot of work.

Oh dear lesser gods, no, that would require a lot of digging. I did mean in-universe. I'm of several minds, since there'd be no real reason to not reuse designations or keep records of transfers without "retiring" a number, but the Foundation is also very meticulous.

Then again, the lack of a conclusive answer from this thread kind of does answer my question.

Something I think is worth considering for this question: How many individual D-Class personnel might have come and gone in the history of the Foundation? Without ever recycling numbers they'd eventually be giving, like, seven-digit numbers to new incoming Ds. Maybe it's like phone numbers, with some extra designator (like the equivalent to area codes) but only people in the relevant department would tend to use it.

Or maybe there could be some kind of archival record that cross-indexes D-# designations with dates, and from there the profile details of past D-Class subjects are accessible? So even if a number was reassigned, the records of the previous holder(s) of that number aren't erased.

On the one hand, big numbers would get impractical to use (Researcher X: "D-18005552888, please proceed through the doorway."). On the other hand, the Foundation is unlikely to just completely erase information that may need to be reviewed later ("The D-class from this test last year, did they happen to be born in Texas?" - "They died a few months ago, we deleted their profile.").

I think ultimately it's just a bureaucratic background detail, the specifics of which can change depending on one's headcanon or the setting/context of a particular story.